Oscillatory behavior of the selective catalytic reduction of NO x by hydrogen (H 2 –SCR) was studied over Pt/HY catalyst. It was found that the oscillation frequency increases with the increase of catalyst amount, reaction temperature and the H 2 concentration in the feed gas within the range of tested experimental conditions. The NO x -TPD indicates that the adsorption–desorption of NO x on Pt particles is involved in the oscillation. It is proposed that the primary surface species on the Pt particle is alternatively changed between Pt–NO x and Pt–H, and the oscillatory behavior of the H 2 –SCR is a statistic result of oscillations on many Pt particles. The in situ FTIR and NH 3 pulse injection studies reveal that NH 3 is involved in the synchronous oscillation.